The present invention relates to a method for operating a circuit-breaker, in particular, a vacuum circuit-breaker, using a tripping control device, which independently from the instant that a breaking command is given, initiates the separation or opening of the contact members at an instant which is fixed relative to the zero crossing of the current.
A method of this type has become known from the teachings of U.S. Pat. No. 3,555,354, for example. The aim of this method is to limit, to the greatest extent possible, the duration of the arcing between the contacts of the circuit-breaker and, on the other hand, to ensure a sufficient clearance between open contacts at the instant of current zero crossing. For this purpose, via a transformer, the tripping control device detects the flowing current and acquires periodic pulses from it, respectively, at the zero crossing of the current and at the maximum or minimum of the current waveform. Both pulses are fed via a timer to an AND element, which in addition can receive a signal derived from the absolute level of the current. In a conventional way, the tripping signal output by the AND element operates a tripping solenoid, which actuates a valve or a latching arrangement to release the tripping mechanism or the breaker mechanism.
In the same way as certain gas-blast circuit-breakers, vacuum circuit-breakers have the property whereby after a current interruption, their contact-break distances attain a high dielectric strength in an extremely short period of time. Therefore, especially in strongly inductive circuits, they tend to have so-called multiple re-ignitions, which represent a rapid succession of arcing occurrences between the opened contact members. High overvoltages can be associated with this process. Moreover, in three-phase systems, due to multiple re-ignitions in the first-quenching electric pole of the circuit- breaker, a virtual current chopping can occur in the last quenching electric poles of the circuit-breaker, through which means overvoltages are likewise produced.
To avoid such overvoltages, attempts have already been made to insert contact materials in vacuum circuit-breakers. Due to the relatively high vapor pressures of individual components, these contact materials sustain an arc as close to the zero crossing of the current as possible. However, opposing this advantageous property is a reduced capability for high switching capacities. The result is that it is difficult to produce a circuit-breaker, which is suited for high switching capacities and, at the same time, avoids the formation of overvoltages.
Furthermore, it is generally known to avoid overvoltages which occur, particularly when actuating motor circuits, by means of voltage surge protectors or combinations of resistors, capacitors and inductors with similar properties. Apart from the difficulty of accommodating such elements at a location in a circuit arrangement suitable to ensure their effectiveness, these components must also be individually adapted to the properties of the respective circuit at hand.
A switching method is known with the aim of avoiding the difficulties described in the preceding, whereby two of the contact-break distances of a three-pole circuit-breaker are opened later than the first contact-break distance; that is later by at least one third of a cycle of the mains frequency plus the minimal arcing time in the first contact-break distance (DE- C-28 54 092). This method in principle prevents the occurrence of the so-called virtual current chopping in the two last-quenching electric poles of the circuit-breaker. Due to the fact that the switching operation can begin at any instant, the multiple re- ignitions in the first-quenching electric pole, which likewise are the cause of overvoltages, are not able to be prevented.
If a circuit-breaker is operated with the application of a tripping control device, then in principle it is possible for one to undertake switching operations without overvoltages in three-phase systems. These switching operations are possible when in the case of the controlling system, a clearance exists between the contact members at the time of the current zero crossing of all electric poles of the circuit-breaker, said that the arc cannot ignite again under the influence of the transient recovery voltage. Such a switching method proves to be extremely difficult to implement, because the so-called opening window, that is the time interval in which the contact members must be opened, only has a range of 2 ms in a mains with a frequency of 50 Hz. Conventional circuit-breakers are not able to execute an opening operation with such precision. Moreover, the mechanical properties of circuit-breakers can change during their utilization period to the extent that after a long operating period and altered environmental conditions, the circuit-breakers are no longer capable of maintaining the opening window, even if they had originally been suited for this when new.